As is well known, NTSC color television signals have interleaved luminance and chrominance information that is separable by use of bandpass filters and the like. Decoding of the interleaved chrominance and luminance information to produce R, G and B signals introduces crosstalk between the chrominance and luminance information that limits the quality of the resultant display. Processing of the combined signals by means of conventional bandpass filters has also generally limited the bandwidth of the recovered signals. More expensive comb type filters have not been used extensively in receivers, although some performance improvement is obtainable.
Conventional video cassette recorders (VCRs) have a limited frequency response (about 2 MHz) due to the characteristics of the carrier frequency used to modulate the luminance information for recording. This limitation has effectively precluded use of conventional VCRs to record Teletext information, which comprises encoded data transmitted on selected horizontal lines in the vertical blanking interval of the television signal.
Recent improvements in the frequency characteristics of recording machines resulted in a so-called super VHS VCR having a bandwidth of about 4 MHz. This wide band VCR is equipped with separate luminance (Y) and chrominance (C) signal inputs as well as a composite (mixed Y and C) video input. Signals that are developed and maintained in separate luminance and chrominance form may be recorded and played back with much improved fidelity. Such VCRs incorporate comb filters, for processing both composite video signals and separate Y and C signals, with little restriction on bandwidth. Thus the luminance signal can have a full 4 MHz bandwidth with a super VHS VCR. The separate Y signal may even be comb filtered in the VCR to remove interference.
The large bandwidth capability of wide band VCRs implies the possibility of recording Teletext data, which as mentioned, has not been possible with conventional VCRs. The use of time domain filters of the comb type, however, corrupts the Teletext data because of the alternate horizontal line additions and subtractions performed in the comb filtering process.
The present invention broadly covers a video processing system for television signals bearing Teletext data on horizontal lines that are subject to processing by comb filters and has specific application to wide band VCRs.